Method of automatically calibrating a microprocessor controlled digital multimeter

ABSTRACT

A new microprocessor controlled digital multimeter offers significant advantages to the user. Automatic calibration and correction routines ensure long term stability and simplify maintenance; self diagnostics greatly help troubleshooting and even warn of impending failures; software implementation of logic design simplifies hardware, improves reliability and adds capabilities such as fast auto ranging and software I/O control; the ability to process data permits averaging, linearization and normalization of input measurements, minimum/maximum storage and limit detection.

CROSS-REFERENCE TO RELATED APPLICATION

This is a continuation-in-part of application Ser. No. 710,218 filedJuly 30, 1976, now abandoned in the name of the present inventors andassigned to the present assignee.

BACKGROUND OF THE INVENTION

The present invention relates to a method of automatically calibrating amicroprocessor controlled digital multimeter and more specifically to amethod where the system includes self-calibration and diagnosticfeatures.

In the calibration of existing multimeters zero and full scale manualadjustments are made periodically (one week to three month intervals) byuse of control knobs and a meter indicating a null condition. Thisroutine is time consuming and complex and does not ensure long termstability. In addition if a parameter or component is seriously outsideof tolerable limits the user has no simple diagnostic tool fortroubleshooting.

OBJECTS AND SUMMARY OF THE INVENTION

It is a general object of this invention to provide an improvedmultimeter.

It is a more specific object to provide a multimeter which hasself-calibration and diagnostic features.

It is another object of the invention to provide a multimeter which hasimproved autoranging.

In accordance with the above objects there is provided a method ofcalibrating a microprocessor controlled digital multimeter. The error ismeasured in substantially all modes and ranges for substantially allzero and fullscale points between a voltage reference and the actualoutput of the multimeter including making a plurality of internalself-calibration measurements. The results of such measurements arestored as constants. It is determined which of such constants arerelevant with respect to each of the modes and ranges. The relevantconstants are utilized for providing digital correction for each of themodes and ranges.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified block diagram of a multimeter embodying thepresent invention;

FIG. 2 is a circuit schematic of a portion of FIG. 1;

FIG. 3 is a simplified flow chart of the microprocessor program used inthe present invention; and

FIG. 4 is an elevation view of the front panel of the microprocessor.

Table I shows the derivation of modified correction constants;

Table II shows the universal correction equation and definition of the Xand Y constants; and

Table III is a switch setup table.

SYSTEM DESCRIPTION

FIG. 1 shows a simplified block diagram. The Input Attenuator, Modeselect, AC and Ω Converter and the Buffer, convert the various inputsignals to a properly scaled DC voltage input to the A/D Converter. Thedual slope A/D Converter compares its input to the Voltage Reference andsignals the end of the integrating time. Control circuitry is requiredto select the proper input switching configuraton for each measurement,to generate A/D Converter timing signals and to control the Display.

This control is normally performed by random logic. In the 7115 aMicroprocessor is used as the Control Unit. The inputs and outputs ofthe processor are formatted and sequenced by shift registers to simplifythe interface. The major control tasks performed by the processorinclude setup of input switching and dual slope timing.

The microprocessor system accepts range and function control signalsfrom the front panel or remote control (i.e., per IEEE 488 standard) andfrom a ROM lookup table supplies serial data to the range and functionswitch controlling shift register. These data are decoded and used todrive the switches shown in the analog switch system described later.

AUTO CALIBRATION

System accuracy depends on many factors. Each tap on the resistornetworks which control attenuation factors, gain and reference current(Ω mode) contribute error.

The Voltage Reference, with which all inputs are compared, creates anadditional error. The number of error-contributing components describedabove is typically 33 in a conventional precision DMM.

    ______________________________________                                                    Precision         Reference                                                   Resistors                                                                             Switches  Elements                                        ______________________________________                                        AC Atten and Conv.                                                                          6         2                                                     DC Atten and Buffer                                                                         5         2                                                     Ω Converter                                                                           7         4         1                                           A/D Converter 1         4                                                     Voltage Reference                 1                                             Total       19        12        2                                           ______________________________________                                    

Clearly, any technique to reduce the number of critical components isuseful.

The use of Autocalibration as described below in the 7115 reduces thenumber of critical components to 5, one Voltage Reference and 4resistors.

The autocalibration cycle is initiated after every 100th or 1000measurement or just on external command. There is also one autocalibratecycle when power is turned on. Autocalibration performs 16 measurements,which cover (after eliminating duplications) all zero and fullscalepoints of all modes and all ranges. These measurements are taken with ahigh stability Zener and a resistive divider as the references includedin the block labeled "Voltage Reference Divider and Ohms Standard".Since these components are removable they may be replaced with a freshlycalibrated pair, thereby eliminating any down time for the instrumentwhile being calibrated. In prior art instruments all circuit boardswould have to be replaced which is, of course, impractical. The internalreference elements potentially can also be replaced by external ones. Inthis case the instrument will never need recalibration. Specifically,the "voltage reference" is typically 10 volts which is divided down bythe three precision resistors having values of 9 K, 900 and 100 ohms,respectively. These resistive standards therefore divide down to 1 and0.1 so that the gain of the buffer amplifier and ac converter can bedetermined at different levels.

When the Autocalibration cycle is initiated the instrument automaticallymakes 16 (A through S) internal autocalibration measurements. Thesemeasurements after modification are stored as constants A₁ through S₁ ofTable 1 in RAM memory.

The Universal correction equation is:

    R.sub.CORR +[±(|R.sub.UNCORR |-200)-X]Y,

where R_(CORR) is the corrected and R_(UNCORR) the uncorrected reading,and X and Y are variables derived from the 16 constants as a function ofmode and range according to Table II. Complete information is given tothe reader in FIG. 2 and Table IV for checking the validity of theequations shown. In the correction equations the (1+E)⁻¹ ≃1-Eapproximation is used, where E<10⁻³. The resulting error is less thanthe resolution of the DMM. The subtraction of 200 counts abovecompensates for an intrinsic offset in the A/D Converter.

By necessity the AC corrections are the most complex. There we correctfor DC gain, DC offset, and AC gain and offset of the RMS Converter. Thenear-zero non-linearity of the converter is not corrected for, but iskept within the AC accuracy specification of the DMM. On the other hand,if higher accuracy is desired, the non-linearity can be measured, and bythe polynominal transformation feature, compensated for.

DIAGNOSTICS

Storing of the calibration constants is necessary not only forself-calibration but also for fault diagnosis. Each modified constant A₁to S₁ has a predetermined range as shown in Table IV. This is themaximum expected drift in the constants due to aging, temperature andother factors. If the constant exceeds the preset range, a diagnosticroutine will indicate it by turning on a warning light. It is importantto note that the instrument is capable of fully correcting for errorsseveral times larger than the preset range. Nevertheless, if the errorexceeds the normally expected maximum value, the cause should beinvestigated. The diagnostic light, therefore, could be considered anearly warning signal showing anticipated failures as well as actual.

                  Table IV                                                        ______________________________________                                        Diagnostic Warning Limits of the Correction                                   Constants as Percent of Full Scale                                            ______________________________________                                        A.sub.1                J.sub.1                                                B.sub.1                K.sub.1                                                C.sub.1     ±0.2%   L.sub.1   ±0.2%                                     D.sub.1                M.sub.1                                                E.sub.1                N.sub.1                                                F.sub.1                P.sub.1                                                G.sub.1     ±0.2%   R.sub.1   ±0.7%                                     H.sub.1                S.sub.1                                                ______________________________________                                    

If the warning light is on, the first step is to remove the cover andoperate the diagnostic switch. This calls up the diagnostic displayroutine, and at the push of an advance button displays the calibrationconstants on the normal readout, one by one. A trouble-shooting chartdirects the operator to the failed or suspected board or component.Example; N₁, P₁, R₁, S₁, are out of range: defective RMS converter. Thedisplay feature has one additional use as well. During routinemaintenance the constants can be inspected and, if close to a limit,even the anticipatory warning itself can be anticipated.

There are other diagnostic aides in the instrument in the form of 34 LEDindicators to indicate the operational status and possible failure ofother specific components. These additional LED status lights, thoughsome are under software control are not evaluated by the software, i.e.,they are not influencing the operation of the warning light. These LED'sare additional visual troubleshooting aids with the help of charts. Theyprovide continuous parallel display eliminating the need for most scopeand DVM probing.

Most of the LED's are used to show which relays and FET switches areenergized or are on. An early procedure in the diagnosis of aninoperative unit, and following review of the constants, is a review ofthe actual switch configuration against the setup table (Table III).Discrepancies would indicate (1) failure of the transmission of selectedrange and function data to the CPU due to system I/O or input shiftregisters, (2) incorrect transmission of data to the range and functionshift register due to optical couplers, ROM lookup table, shiftregister, or the relays.

One LED indicates presence of the cycle advance (or system timing) clockand its synchronizing pulse.

Another LED is used to confirm sampling of measurement of the inputsignal. The last LED displays the status of the high frequency clockgate used to measure time in the dual slope A/D Conversion system.

The lack of an indication of the last three LED's indicates failure ofthe CPU, ROM, or specific logic circuit board assemblies.

SWITCHING SYSTEM

The switches shown in FIG. 2 are used to set up normal measurements inall functions (i.e., VAC, VDC and Ω). They provide proper connection tovarious input Converters and Buffers. Attenuation and gain factors arealso selected with these relays. They also configure the instrument toperform Autocalibration measurements. The switch setup table, Table III,defines the setup for each range and measurement mode.

Relays K6, and 7 set DC Attenuation factors of 1 and 1/100. Relays K9and 10 select gain settings of 1, 10 and 100 for the input Bufferamplifier. Relays K16, 17, 18, and 19 set the reference current forresistance measurements. Relays K17, 18, and 19 are also used to controlthe attenuation factors for the AC Converter. (1/10, 1/100 and 1/1000.)

A measurement of 100 VDC would dictate a setup as follows: Relay K4connects the input to the DC Attenuator. K7 is closed to set up the ÷100ratio tap and connect the IV signal to the Buffer amplifier. The gain isset to ×10 with K10. The amplifier output is connected to the A/DConverter.

Measurements in the AC Voltage mode are made in a similar manner withK11 connecting the input to the AC Attenuator and K12 which connects theAC Converter output to the input amplifier.

These relays also connect the internal voltage and resistance standardssequentially to the input conditioning circuits. The resultant measureddata is ultimately used to correct subsequent measurements.

Relays K21, 22, 13, 14, 15, 1, 5, and FET switches S2, and S3 are usedin the calibration of the AC and Ω Converters DC Attenuator andAmplifier. See Table IV for the relay setups used in the Autocalibrationcycle.

S2, and S3 connect the 1.0 V and 0.1 V references to the input amplifierto measure the gain factors.

K13, 1, 4, and 7 set up the calibration of the DC Attenuator. The +10 Vreference is applied to the top of the attenuator. The attenuator outputvoltage is normally +0.1 V. K9 is used to provide adequate gain tomeasure the attenuation factor. DC Offset is measured in the 0.1 V and 1V ranges by closing K7 in conjunction with K9 and 10. This connects theamplifier input to signal low. The resultant output then representsamplifier offset. The reference divider is also used as the resistancestandard for calibration of the Ω Converter. The 100 kΩ resistor in theDC Attenuator is used to calibrate the higher ranges.

Briefly, the above techniques are used to relate all previously criticalcomponents to a simple standard. The correction equations discussedelsewhere inpart a high level of accuracy to all measurements in spiteof drift and long term aging of most of these components.

FLOW CHART

For general overview FIG. 3 shows the simplified flowchart of the 7115.There is an automatic power on reset. From Start the program checks forany Autocalibrate request. After a preset number of measurements or onexternal command an Autocalibrate cycle is performed. Calibrationconstants are measured and stored and self diagnostics is performed.Subsequently all the remote or local mode, range, control and optionconditions are loaded into RAM memory. After that, all the requiredrelay and control flip-flops are set under software control. The nextcycle is Measure. There is on Overflow check and if we are in theAutorange mode, depending on the size of the overflow, we change therange one up or to maximum.

The purpose is to optimize autoranging. A slow drift to overflow islikely to fall into the next range, but a severe overload causingsaturation of the amplifier is best measured first at maximum range.This, as will be shown later, ultimately results in the fastestautoranging. If the overflow is occuring without autoranging or alreadyon the maximum range, the Overflow display routing is called. Note thatoverflow readings are not corrected.

If no overflow occurs, the hardware counted least significant digits ofthe reference integration measurement are loaded into RAM. This is notshown in the simplified figures.

If on the right range, that is, readings between 1X and 0.10X or onminimum or Hold range, then Correction test follows. If the reading isless, the software loop shifts down range once or several timesaccording to the size of the reading. Since we have 51/2 digits ofinformation, even from maximum range the instrument can shift downexactly to the correct range in one step. As we have seen, upranging isachieved also in one step if it is certain that the reading is on thenext scale up, or in maximum two steps, if severely overloaded. First amaximum range measurement is taken and then the instrument, as justshown, down ranges, if necessary several times, but always to the rightrange without taking any additional measurement. If a measurement istaken, we have the choice by an internal switch to display theuncorrected reading or to correct the value by the correction equation.The decision is made at Correction-test in the flowchart. Aftercorrection, the Data processing test is performed to see wether any orall of the optional data processing functions are required. Afterprocessing, if any, the net result of the measurement with correctionfactors applied is displayed by means of a display shift register andLED readouts. The data in optional parallel or serial format are alsopresented to the interface bus. Subsequently if no diagnostic display isrequired a new measurement cycle can start.

The described program occupies 2.5 kbyte of ROM not including 1.5 kbytefor the optional data processing functions.

Referring now to FIG. 3, the simplified flow chart of the microprocessorprogram, each of the flow chart blocks will now be described in detail.Such description will enable a skilled programmer to easily implementthe flow chart in the language of the INTEL 4004 microprocessorprogramming language.

The instrument is initialized at turn-on, which is START, and the firstthing that is tested is to determine whether an autocalibration routineis required; that is, is it necessary to determine all of the correctionconstants and store them in a RAM memory. This is done at turn-on andperiodically based on the customer's preference or is remotelycontrollable.

The decision to autocalibrate sets up the relay patterns in Table IV.The instrument then is sequenced through the steps (under the headingFACTOR) A through S setting up relays as noted. The data resulting fromeach of these measurements is stored in RAM memory. These sixteenmeasurements serve to characterize each of the input conditioningcircuits in the instrument; typically (see FIG. 2) the ac converter, theohms converter, the ac attenuator and the buffer amplifier gain in eachof its gain settings. Following the autocalibration cycle the instrumentsenses the control signals which set up the measurement function andinitializes on one of the ranges.

A measurement is then taken and is noted in the MEASURE block. Thatmeasurement is tested for overflow. If the measurement is, in fact, anoverflow reading, then MAX RANGE OR HOLD determines whether or not thisis the highest range and therefore an unavoidable overflow, and if so,the OVERFLOW SIGN is indicated. If it is not on the maximum range or onrange hold, then SMALL OVERFLOW will cause the instrument to up range byone range and a substantial overflow will cause the instrument to rangeto the highest range; for instance the 1,000 volt range ac. If it hasbeen necessary to follow the foregoing loop in the flow chart then theinstrument makes another measurement and performs the same test for theoverflow condition which should no longer exist. If the reading is onscale, that is, not overflow, then it will test for underflow; in otherwords, is the reading too low to present an optimum reading withoutoptimum resolution. If the reading is too low MINIMUM RANGE or HOLD istested. If this condition is true, and therefore the range could beoptimized by making it a more sensitive range, that range is directlydetermined by the reading just taken set up in the block SET CALCULATEDRANGE and a measurement then is initiated again.

Summarizing the autoranging capability of the present invention if thereis an underflow, the amount of underflow is sensed quantitatively andthe proper range immediately directly selected without any intermediatesteps. In the case of overflow a small overflow can be sensed to uprangeone step as shown in the flow chart. If it is a large overflow, thiscannot be directly sensed and thus the uprange is set to the maximum.Then a subsequent measure step is conducted where underflow will occurif the max range is not proper and the proper range may be directlyselected. Thus in most cases as discussed above the proper range can bedirectly selected without intermediate steps.

Presumably all conditions are now met and it is possible to go throughthe flow chart for MEASURE, OVERFLOW, UNDERFLOW and down to CORRECTIONREQUIRED. Correction is normally required and is only defeated fortroubleshooting purposes. In the correction required routine, each ofthe readings is stored in RAM memory and modified based on equations anddata shown in TABLE I and II. This operation is performed within themicroprocessor system in a fairly straightforward manner utilizingnormal data manipulation routines.

The correction equation of TABLE II is actually the revised format ofthe classical equation Y=mX+b; however, it is in the form X=(Y-b) (1/m)where the 1/m term is equivalent to Y and b is equivalent to X. Thus, Xis an offset term and Y is a slope term.

When the data is corrected, the next question is DATA PROCESSINGREQUIRED. And that is, simply, does the data need to be linearized basedon special application programs, does the data need to be stored at highor low readings, does data need to be compared against high or lowlimits, etc. This is the time at which the data is manipulated ormonitored for those purposes. Then the data is displayed either throughthe front panel display or remotely.

Another troubleshooting aid is the ability to display certain diagnosticdata. Principally that is the display of the correction constantssupplied in TABLE I. After these constants are displayed the beginningof the flow chart is returned to for another measurement. Specificallywith relation to the diagnostic display, the constants A₁ through S₁when displayed are compared against prescribed limits and error isindicated in any particular constant by a marked variation from thelimit.

For example, one example of the use of the diagnostic routine anddisplay is as follows. Assume that the 100 volt range of the acconverter is not functioning properly and it erroneously reads the sameas the one volt range. This would first of all show up as a major errorin the constant R₁. A major error in the constant R₁ would indicate thatperhaps relay 17B has failed or the resistor associated with that relay.

Furthermore if all of the ac constants are well outside of their limitsit would be a clear indication that either the input was not connectedto relay 11 or the ac converter output was not connected through relay12 or the failure of the ac converter itself is indicated. Anotherexample of the use of the modified correction constants as diagnostictools is if a certain group of constants are all out of limit. Forexample if H₁, which is the 100 ohm scale factor correction constant,and A₁ the 0.1 volt dc gain constant, and furthermore the constants J1,K1 and L1 have all significant errors one can infer that these errorsall relate back to one specific region; that is, since they all employthe use of relay 9 in the buffer amplifier and the 1Kohm resistorconnected to it, it would indicate that a failure of one of those twocomponents or others in that area are the source of the difficulty.

Thus by the foregoing diagnostic technique the failures can bespecifically pinpointed to one sector of the circuitry such as thebuffer amplifier as discussed above.

CONCLUSION

We discussed some of the features of a Digital Multimeter, which for thefirst time in a single instrument, as opposed to larger systems, iscapable of full self-calibration based on its own or externalreferences. The instrument also monitors its calibration constants, andin case of unexpected drift, warns by a light of actual or anticipatedfailure of specific components or sub-assemblies.

What is claimed is:
 1. A method of automatically calibrating amicroprocessor controlled digital multimeter comprising the followingsteps: measuring the error in selected modes and ranges for two pointsbetween a voltage reference and the actual output of the multimeter atsaid two points including making a plurality of internalself-calibration measurements, storing the results of such measurementsas constants, determining which of such constants are relevant withrespect to each of such selected modes and ranges, and utilizing therelevant constants for providing digital correction for each of saidmodes and ranges.
 2. A method as in claim 1 where the multimeterincludes a plurality of resistors corresponding to different modes and aplurality of associated switches and where said step of making saidplurality of internal calibration measurements includes the actuation ofunique combinations of said switches to provide said constants.
 3. Amethod as in claim 1 including the step of deriving diagnostic warninglimits from said constants by comparing said constants with apredetermined range.
 4. A method as in claim 1 where the multimeter hasan autoranging capability and including the step of switching directlyin one step to the proper range.
 5. A method as in claim 1 where saidtwo points are zero and fullscale.
 6. An automatically calibratedmicroprocessor controlled digital multimeter comprising: means formeasuring the error in selected modes and ranges for two points betweena voltage reference and the actual output of the multimeter at said twopoints for providing a plurality of internal self-calibration correctionconstants; means for storing said constants; means for utilizingselected constants for correcting a measurement made in a selected modeand range; said means for measuring said range including said voltagereference and a plurality of reference resistors forming a voltagedivider all of which are physically removable from said multimeter forrecalibration.